It is known that optical representation of data, such as machine-readable bar codes, logos, etc., can be attached, placed or marked on products by various means, such as industrial ink jet printing, electrolytic chemical etching and laser markings. The data can serve multiple purposes including product identification, track and trace information, anti-counterfeit detection, etc. Laser-based marking devices do not require inks, solvents and other chemicals and thus can provide a marking implementation that is comparatively less expensive with lower operating costs and more environmental friendly, such as without generating hazardous solvent emissions. Moreover, the laser-based markings are generally longer lasting, have better resolution quality and do not wear off easily.
A majority of presently available laser-based marking systems use galvanometer-based optical scanning technology where a laser beam is scanned across the object to be marked. Although the technology has made advances in terms of speed and performance, placing a 2-D bar matrix code or a high resolution image on the object can be challenging. For example, placing a high resolution image with an example resolution of 1024 by 768 pixels would require 786,432 marking operations in a galvanometer-based system. On the other hand, a laser marking system based on a Spatial Light Modulator (SLM), such as a Micro-Electro-Mechanical-System (MEMS)-based digital micromirror device (DMD) or Liquid Crystal Devices (LCD) based SLMs etc., can simultaneously process a complete code matrix or a high resolution image in a single operation. The ability of these devices to project variable data at high speeds make them an excellent choice for laser marking systems performing serialized data bar coding specially two-dimensional (2-D) data matrix codes. However, certain drawbacks can arise during their operation. In SLM based marking, the total energy/power of the laser is shared among all the pixels as against a scanner based system where all the power is focused at one pixel. Thus, in order to mark the intended the target, the overall energy/power of the laser has to be higher. However, the overall power cannot be increased indefinitely and is limited by the damage threshold of the SLM. Several approaches have been proposed that try to address these drawbacks, but with limitations.
One prior art laser marking system (U.S. Pat. No. 6,836,284) is believed to use a digital micromirror device (DMD), operating as a SLM that requires a beam expansion and beam contraction mechanism (i.e., requires optics adapted to intentionally affect the size of the cross-section of the beam) to avoid damage to the DMD. The beam expansion spreads the optical power of an incident beam over a larger area and thus reduces the irradiance (power per unit area) so that the DMD is not damaged. After reflection from the DMD, the beam is contracted again to increase the irradiance. The system described in the foregoing patent, however, is somewhat impractical since the physical dimensions and the cross-sectional area of available micromirror devices are relatively small (in the order of few square cm). Due to their small cross-sectional area, the present Applicant believes that the spatial profile of the incident laser beam cannot be expanded beyond a certain magnification limit (L), as shown in FIG. 14. That is, the irradiance of the incident laser beam cannot be reduced by a factor greater than L. Any further magnification (M>L) would result in part of the beam to miss the device. Thus, in practice, Applicant believes that the marking system proposed in the foregoing patent would not be effective for applications requiring optical intensities L times higher than the DMD damage threshold. Also, in applications where large sized marks are required, with mark size being comparable to the size of a DMD/SLM itself, Applicants submits that the beam expansion and contraction mechanism would be useless.
Another solution suggested in prior art to avoid damage to the SLM, as disclosed in U.S. Pat. No. 7,058,109, appears to involve standard lasers and amplifiers, which produce optical beams characterized by either a Gaussian irradiance distribution or other non-uniform distribution. As a result, the irradiance of the laser beam at the target surface would be Gaussian or non-uniform, which could adversely affect the quality of the marks, whether non-ablative or ablative. For example, in the case of marks based on ablative marking (as may involve a metal foil), the relatively high irradiance at the center of a Gaussian beam, as compared to substantially lower levels at the outer edges of the beam, could result in a melting of the foil at the center of the mark leaving a hole, whereas the irradiance level at the beam edge may not be sufficiently high to ablate the foil metal. For example, in the case of non-ablative marking (e.g., photo-chemical change of a coating under laser irradiance), the relatively high irradiance at the center of the Gaussian beam might lead to heat conduction to areas on the target that are not designated for marking, and the relatively weak irradiance at the outer edges of the beam may not be conducive to a color change. If a 2-D data matrix code is marked on a target (ablative or non-ablative) the non-uniform irradiance distribution would ruin the contrast ratio of the code and a code reader would be unable to read it. Thus, the irradiance distribution across the laser beam cross-section should be sufficiently uniform so that the information, i.e., the projected image is within acceptable quality levels, e.g., readable marks without loss of information. Accordingly, it is desirable to provide a practical and reliable SLM based laser marking system that provides a cost-effective solution to overcome the above-described issues.